1. Field of the Invention
The present invention relates generally to wireless positioning techniques, and more particularly to a system and method for determining the position of a target device by ordered transmission of wireless signals even without distance information and pre-synchronization among beacon devices. The beacon device is a device which is the basis of determining the position of the target device and the position of which is previously known before the process of determining the poison of the target device is performed.
2. Description of the Prior Art
Recently, as the advantages of UWB (Ultra Wideband) wireless technologies are being watched, researches for short-distance high-precision wireless positioning systems have actively been in progress as one of applications of UWB pulses having extremely short durations.
Because the UWB pulses have the extremely short durations below nanosecond, they have been applied to the positioning systems so as to achieve a high-precision wireless positioning at centimeter levels, which could not be obtained through the existing communication systems. Additionally, the UWB pulses have extremely low power levels through the ultra wideband, and thus they can remove interference with other electronic appliances.
The existing wireless positioning techniques disclosed in U.S. Pat. Nos. 5,589,838 and 5,216,429 require pre-synchronization among beacon devices. Because in the prior art, the precision of positioning determined by the time resolution is generally at levels of about several tens of meters, it is possible that the systems operate in a range over which errors occurring in a process of synchronization among the beacon devices do not greatly damage the precision of positioning.
However, in the currently required systems of a high precision, e.g., having time resolutions below nanosecond, PLL (Phase Lock Loop) adjustment errors for obtaining the time synchronization, wired signal transfer errors, etc., become greater than the pulse durations if the systems are constructed so that they require the time synchronization among the beacon devices as in the prior art. This causes severe problems due to the synchronization errors when links among the beacon devices for the synchronization are made through not only wireless paths but also wire paths.
As a consequence, if the existing systems are applied as they are, it becomes impossible to achieve a high-precision positioning that makes the best use of the advantages of the UWB pulses having the extremely short durations.
In addition, U.S. Pat. No. 6,054,950 describes methods for determining the position of a target device using previously known coordinates of beacon devices without the necessity of data transmission/reception through wire connections among the beacon devices or pre-synchronization. One of the proposed methods is a method for synchronizing the respective beacon devices using GPS. However, because this method requires a GPS receiver, etc., (provided in the respective beacon device) the system is complicated and thus it is difficult to put the method to practical use. Additionally, because the system using the GPS has limited precision of time synchronization, it is impossible to achieve the wireless positioning of high precision below several meters through this method.
According to another of the proposed methods, the second beacon device and the target device receive the first pulse generated from the first beacon device, and the second beacon device successively transmits the first pulse to the third beacon device and the target device. This process is repeated up to the N-th beacon device, and the position of the target device is calculated using the distances among the beacon devices calculated using the differences among arrival times of the respective pulses received by the target device and the previously known coordinates of the beacon devices.
However, this method has the problems in that because the target device should serve as a signal receiver and processor, the structure of the target device becomes very complicated, and thus the application of this method is limited. Additionally, in order to perform the precise positioning of the target device by applying this method, the assumption that the distances among the beacon devices calculated using the previously known coordinates of the beacon devices are the same as the lengths of the actual wave propagation paths among the beacon devices should be satisfied. However, this assumption may not be satisfied because the actual wave propagation paths may be complicated according to diverse environmental variables such as the curved surface of the earth and so on, and thus the synchronization among the beacon devices is ultimately required in order to heighten the precision of positioning even though this method is used.
In addition, the target device (or tag) that is an object of the position tracking is generally a portable simplified individual device to which the power supply is not easy. For example, in order that the target device can widely be applied to various application fields from the personal portable communications adopting the short-distance wireless positioning system to the tags for logistics managements in RFID (Radio Frequency Identification) systems, it should generally be designed to have low power consumption. Although the UWB pulses have extremely low power levels, it is strongly necessary to develop a position tracking system that enables low-power operation using a target device of a simplified structure so that the number of operations and the number of signal transmissions performed in the target device can be minimized.